On Sunday, a United Airlines gate agent stopped two girls from boarding a flight from Denver to Minneapolis for wearing leggings, according to a live account of the debacle by Shannon Watts, a fellow passenger and founder of Moms Demand Action, a grassroots movement to end gun violence. "She's forcing them to change or put dresses on over leggings or they can't board," she tweeted. "Since when does @united police women's clothing?

Responding on Twitter, United Airlines, a unit of United Continental HoldingsUAL, +0.64% , tweeted that such decisions were at the discretion of a gate agent and said it has the right to refuse transport for passengers that violate Rule 21 of the airline's contract of carriage. That rule says passengers may be denied if they "are barefoot or not properly attired." It does not specify what constitutes proper attire; Watts noted that the father of one of the children was wearing shorts.

Later Sunday, United tweeted at Watts again: "We appreciate you being our eyes and ears. The customers this morning were United pass travelers." It added, "There is a dress code for pass travelers as they were representing UA when they fly." That code states such travelers should be "well-groomed, neat, clean and in good taste." United pass travelers typically travel on discounts and comprise current and retired employees and their families. (United did not immediately respond to a MarketWatch request for comment.)